In the interest of maintaining confidentiality, let’s just say I’m a tenured professor at a top liberal arts college, and was previously on the faculty at a state flagship, so can speak to the learning environment at a few kinds of institution.
OP: Having taken Calculus BC as a junior means your background is stronger than average, but wouldn’t be all that unusual at my institution or any of those mentioned above. Depending on what you decide to do next year you would likely begin with an “early intermediate” course like linear algebra, and wouldn’t run out of courses.
That said, I’ll give you a bit of advice that I like to give my students: you should conduct your education as if you will run out of courses. By that I mean your goal in learning mathematics should first and foremost be intellectual independence and self-sufficiency. Students who truly achieve those goals are the best qualified for graduate study in mathematics, regardless of the fancy list of courses they’ve been attending.
It is a bit premature to be having the “top graduate school” discussion. Despite what other posters seem to believe, there is no undergraduate institution that will, on its own, either ensure or prevent admission to a pure math PhD program. Far more important is the quality of work that you do, and your development as a learner of mathematics. Why would access to graduate courses at Harvard matter if you are unhappy with the learning environment, and therefore not doing your best work? How much would the small classes and personal attention at Williams benefit you if you’re unhappy (say) with the isolation of a rural environment? It’s quite hard to compare institutions against each other in an “all else being equal” exercise, when all else is almost never equal.
A great many liberal arts colleges and smaller universities have been mentioned above where you would be well-served. As for specific suggestions off the top of my head:
Any of the upper tier of LAC would be great, a few especially successful programs are: Bowdoin, Carleton, St. Olaf, Williams, Swarthmore, Kenyon, Smith.
Some smaller universities with strong undergraduate programs: Vanderbilt, Wash U St. Louis, Rice, Dartmouth.
Some great state flagships with strong undergraduate math programs: UVA, U Wisconsin, U Texas at Austin.
You may be surprised to learn that, within mathematics departments, you’ll find more similarities than differences in the undergraduate experience among these various institutions. I encourage you primarily to seek out the kind of learning environment that suits you best, in the sort of place where you’ll be most content and ready to do your best work. From there, you can make fine distinctions about curriculum and how best to pursue your post-graduate goals. All of the institutions in the range you are considering will be fine choices from a purely academic point of view.